1. Field of the Invention
The present invention relates to a process for preparing, starting from a slag material, a filler for use in construction materials which contain a hydraulic or bituminous binding agent. The construction materials are in particular asphalt or hydraulic mortar or concrete compositions.
2. Description of the Background of the Invention
Fillers are used in several construction materials. The different types of asphalt, such as asphalt concrete, pouring asphalt, draining asphalt and split (stone) mastic asphalt (SMA), contain for example amounts of filler ranging between 4 and 23% by weight. Fillers can also be added to hydraulic mortar or concrete compositions, in particular to self-compacting concrete compositions, to improve the fluidity of the fresh mix.
Self-compacting concrete (SCC) compositions are hydraulic concrete mixtures characterised by a high fluidity, making vibration unnecessary for placing and compaction. SCC compositions flow instead of slumping, filling even complicated formwork with dense reinforcement. The hardened concrete is particularly dense and homogeneous, giving it particularly good strength and durability. SCC compositions comprise a particularly high ratio of filler and have a high resistance to segregation. Their high fluidity is obtained using superplasticizer admixtures, in particular polycarboxylates, whilst limiting the water content to a minimum to maintain the strength of the cured concrete and to avoid segregation of the mixture.
Up to date, crushed limestone has been the main source of filler for hydraulic mortar or concrete compositions and also for asphalt compositions. However, limestone is a natural and non-renewable material. Moreover, other users, such as the food industry, also consume large quantities of this limited resource, further increasing its cost. For this reason, alternatives to limestone, in the form of waste materials, have long been sought.
The European Guidelines for Self-Compacting Concrete, dated May 2005, disclose for example different additions (fillers) which can be incorporated in self-compacting concrete to improve certain properties or to achieve special properties. Calcium carbonate based mineral fillers are described to be particularly suitable for SCC compared with other available materials but, as described hereabove, calcium carbonate (limestone) is a natural, non-renewable material. According to the European Guidelines fly ash would also be suitable but high levels of fly ash may produce a paste fraction which is so cohesive that it can be resistant to flow. Silica fume would result in good cohesion and improved resistance to segregation but it is also very effective in reducing or eliminating bleed which can give rise to problems of rapid surface crusting. Ground granulated blast furnace slag (GGBFS), which is usually over 95% wt. amorphous (due to being rapidly cooled by quenching in water) and which has hydraulic properties, can also be added to SCC but a high proportion of GGBFS may affect the stability of SCC resulting in reduced robustness with problems of consistence control while slower setting can also increase the risk of segregation. Due to its hydraulic properties GGBFS is moreover a valuable raw material for use as cement additive or for the production of cement clinker. Ground blast furnace slag, slowly cooled so as to be majoritarily crystalline, is also disclosed in the European Guidelines as a possible addition to SCC. However, blast furnace slag has also valuable applications as aggregate (blast furnace slag gravel) and may for example be used in building of roads, in civil engineering, in construction of railway track banks, in field arrangements, recultivation, etc. For such applications, the blast furnace slag should preferably be of a high quality, i.e. it should not have been pulverised during the cooling process as a result of an expansive conversion of β-dicalcium silicate crystals into their γ-polymorph (“falling” of the slag). Such γ-dicalcium silicate containing blast furnace slags are therefore less appropriate for being used as aggregate.
Japanese patent application JP 2004-051425 appears to suggest that the remaining β-dicalcium silicate portion in the slag can be used as a cement additive, but it does not disclose how this portion is to be separated from the γ-polymorph. Instead it concentrates on a process for treating the γ-dicalcium silicate so as to use it as a hydraulic cement additive. Moreover, contrarily to what appears to be suggested in this document, crystalline dicalcium silicates in general, and β-dicalcium silicate in particular, do not possess substantial hydraulic properties. The hydraulic properties of the disclosed mainly relate to its amorphous portion and to the adjunction of additional amorphous pozzolanic slag.
In the article “The use of stainless steel slag in concrete”, A. Kortbaoui, A. Tagnit-Hamou, and P. C. Aïtin, Cement-Based Materials, p. 77-90, 1993, it was proposed to use “treated” stainless steel slag (TSSS) as a substitute for sand in concrete mixes. The described “treated” stainless steel slag was relatively fine and also comprised a small portion of a filler fraction (about 18% of the particles were smaller than 63 μm). However, the experiments demonstrated that the amount of TSSS used to replace natural sand was limited by the negative effect on the workability of the fresh concrete. Moreover, notwithstanding the fact that a quite large amount of superplasticizer was added to improve the workability of the concrete, the slump flow was still reduced. This negative impact on the workability of the fresh concrete composition makes the TSSS inadequate for use as filler in concrete and in particular in self-compacting concrete, as defined by the European Guidelines for Self-Compacting Concrete, of May 2005.
Japanese patent application JP 2002-211960 suggested treating a stainless steel slag with a mineralogical stabiliser, so as to at least partially prevent the conversion of β-dicalcium silicate crystals. However, such a process involves substantial costs, both in terms of raw materials (the mineralogical stabiliser) and of installations and energy.
For asphalt an important property of the filler is its water content. In practice, the water content of the filler used for the preparation of asphalt should be smaller than 1% by weight (see for example the European standard EN 13043:2002) and preferably even smaller than 0.5% by weight. Higher water contents would indeed result in the inclusion of water or water vapour in the bituminous mixture which is to be avoided in order to prevent the formation of a too greasy appearance and the risk on segregation or stripping of the mixture during its application. While fillers containing a substantial amount of γ-dicalcium silicate can theoretically be dried down to such a low water content, in practice their water retention is so high that the cost is prohibitive. Moreover, as soon as the material is back in a normal atmosphere and temperature, it starts rapidly absorbing water again, making its use unpractical in any case. The retained water forms a film around the slag particles which prevents a good adherence of the strongly hydrophobic bituminous binder to the particles. Even when the asphalt is laid, water may penetrate into the asphalt and into the filler particles thus causing again stripping possibly leading to rut formation (in road applications) and tear formation.